Diameter decrement by the repeated exchange of solvent water has been reported in a polyelectrolyte hydrogel ionized by sodium acrylate in its swollen state. This behavior has been attributed to the exchange of counterions of carboxyl groups and the formation of hydrogen bonding. In this paper, the formation and destruction of hydrogen bonding in poly(N-isopropylacrylamide-co-sodium acrylate) (NIPA/SA) gel were studied using Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR) at 25 degrees C. Several swollen samples with different swelling ratios were prepared by changing the number of water exchanges at 25 degrees C. It was found that the ATR-FTIR spectra evidently depended on the initial swelling ratio. By sufficient water exchanges, new peaks appeared at ca. 1713 cm(-1) (assigned to the carboxyl dimer) and at ca. 1650 cm(-1) in the amide I region (assigned to a stretching vibration of the C=O group that forms a hydrogen bonding with the N-H bond of a neighboring amide group), and the amide II band shifted to a smaller wavenumber by ca. 7 cm(-1). From the detailed analysis of the IR spectra, the macroscopic polymer network shrinkage was attributed to the formation of three types of hydrogen bonding, that is, between two carboxyl groups of SA, between the carboxyl group of SA and the amide group of NIPA, and between two amide groups of NIPA. Moreover, the IR results indicated experimentally that the reswelling transition of the present system was caused by the destruction of hydrogen bonding on heating. It was concluded that not only the formation by the exchange of water but also the destruction of hydrogen bonding by heating is essentially important to determine the swelling ratio as well as the volume phase transition behavior in this system.